Two electric motors built to be "identical" will have slightly different electrical characteristics; two gear boxes of the same design will have slightly different efficiencies. Other factors, which as temperature gradients and friction coefficient differencies, play a part. Accordingly, a sheet propelled by two consecutive drive rollers will either be drawn taut until slippage or breakage occurs, or else will sag between said rollers until it fouls on some part of the machinery. Therefore a surge device, usually in the form of a "dancer roller", commonly called a "dancer", is interposed, its purpose being to accumulate a short length of sheet, or to pay it out, either as required.
The dancer roller is usually provided in a two- or three-roller construction. In the three-roller configuration, a dancer roller is interposed between a pair of spaced rollers having the material drawn therebetween. In this configuration, the weight of the lower (movable) roller and its appurtenances is carried by a bight or loop of the goods being processed, and any needed further tension must be secured by weights, springs, or fluid cylinders applied to the ends of said lower roller. With the two-roller design, all tension must be so secured. With the three-roller design, the lower roller travels up and down ("dances"), and is guided in a short path. With the two-roller construction, either or both rollers travel horizontally along short closed-end tracks, or else are joined by bars across the roller ends, and travel in an arcuate path about a common central pivot. With both types of dancer rollers, commercially available sensors are located at each travel limit. These sensors signal the appropriate motor to change speed slightly, and start the movable roller back toward its mid range of travel. Both dancer roller configurations readily compensate for the gradual accrual changes in included length of sheet between propulsion points, but both are much too massive to respond to certain high frequency, low amplitude fluctuations that have come into play with the development of high-strength sheet products such as flexible packaging, which is gaining an ever-increasing market.
Examined from the viewpoint of statics, any roller in the dancer assembly may be considered to be a cylindrical beam loaded uniformly along a central portion of its length, but not to the ends. It thus must have walls thick enough to stand the loads imposed on it, and accordingly is quite heavily constructed, and cannot respond to rapid oscillations.
The second type of sheet/tension variation began to become troublesome when mills were being speeded up to meet demand for the high-strength sheet products mentioned above. In the manufacture of primary flat sheet, exact uniformity of thickness is not attainable. For various reasons relatively thicker and thinner areas will occur, which stack up on themselves when wound into a roll, causing "lobes" which, though not obvious, are readily located by measuring. During unwinding, the sheet is paid off at both longer and shorter radial distances from its geometrical center, alternating as much as several times per second. Other factors, such as goods roll dynamic unbalance, off-center chucking, machinery vibration, and sheet harmonics may compound the problems.
It should be noted here that the faster sheet is run through the processing line, the more tensed it must be, in order to expel the air which otherwise would remain trapped between the sheets and the conveying rollers and act as a lubricant, preventing desired sheet travel rate and tracking. Most processes require a taut sheet; trimming and slitting, for examples, cannot be done accurately on slack sheet. The modern approach is to run the sheet at a tension not far below the breaking point. For reasons given above, sheet breakage presently is a continuing and expensive problem.
Efforts to reduce the machine mass involved in present dancer roller design have not been very successful, because, with the manner in which the rollers are loaded, little can be done about their wall thicknesses and consequent massive weight.
Clearly two very different types of tension/included length difficulties in sheet processing need to be dealt with. The first is the gradual accrual type, which can become become sudden with laps or wrinkles in the unwind roll, or rapid speed changes as might happen with inexperienced operators. The second is the high-frequency, low-amplitude type usually accompanying higher processing speeds. The concepts set forth in this Application address both types, and propose to reduce them to acceptable proportions.